1. Field of the Invention
The present invention disclosed in the specification relates to a semiconductor device constituted by using a thin film transistor having a plurality of gate electrodes. Further, the present invention relates to a method of fabricating the semiconductor device.
2. Description of Related Art
In recent years, there has been increased thin film transistors using polycrystal silicon films in semiconductor layers. According to a thin film transistor using a polycrystal silicon film, high speed operation that is faster than operation of a thin film transistor using an amorphous silicon film by two digits or more can be performed since a mobility thereof is large.
Therefore, there poses a problem of hot carrier effect in which hot carriers generated in a channel jump into a gate insulating film and deteriorate a threshold voltage or a mutual conductance.
Conventionally, in respect of the above-described problem, there has been known a thin film transistor of a multi gate type alleviating the hot carrier effect by providing a plurality of gate electrodes and weakening an electric field applied on a single gate.
FIG. 5 shows an example where a thin film transistor of a conventional double gate type is used as a switching element of a pixel matrix portion of a liquid crystal display device.
As shown by FIG. 5, a semiconductor layer 02 and a source wiring 25 form a contact at a source electrode 22. Further, the semiconductor layer 02 is extended while meandering and intersects with a gate wiring 26 at regions 21 and 21xe2x80x2. Further, the semiconductor layer 02 and a pixel electrode 24 form a contact at a drain electrode 23. Portions of the gate wiring at the intersected regions 21 and 21xe2x80x2 function as gate electrodes.
As is an apparent from FIG. 5, the conventional multi gate type thin film transistor is constituted by the gate wiring 26 in a substantially straight line shape and the meandering semiconductor layer 02.
By adopting such a constitution, a distance between the source and the drain is prolonged and therefore, the ON resistance is increased. Further, the resistance of a semiconductor layer is generally larger than that of a metal conductor and therefore, in respect of the semiconductor layer 02 meandering as shown by FIG. 5, the high frequency impedance of the meandering portion is increased which gives rise to deterioration of the element.
It is the object of the present invention disclosed in the specification to resolve the above-described problem.
According to an aspect of the present invention disclosed in the specification, there is provided a semiconductor device, wherein a meandering gate wiring traverses a substantially straight line portion of a semiconductor layer of a thin film transistor by a plurality of times thereby providing a plurality of gates.
According to other aspect of the present invention, there is provided a semiconductor device, wherein a switching element of a pixel matrix portion of a liquid crystal display device is a multi gate type thin film transistor in which a meandering gate wiring traverses a substantially straight line portion of a semiconductor layer by a plurality of times thereby providing a plurality of gates.
Further, the above-described gate wiring is featured in comprising a metal having the resistance smaller than the resistance of the semiconductor layer.
According to other aspect of the present invention, there is provided a method of fabricating a semiconductor device including a step of forming a semiconductor layer having a substantially straight line portion on a substrate, a step of forming a gate insulating film and a metal conductive film above the semiconductor layer, a step of pattering the metal conductive film into a gate wiring, a step of doping impurities to the semiconductor layer with the gate wiring as a mask and a step of irradiating a laser beam wherein the gate wiring meanders and intersects the substantially straight line portion of the semiconductor layer by a plurality of times.
According to the present invention, the gate comprising a metal having small resistance is meandered and therefore, an increase in impedance in the resistance imposed on the meandering portion is small. Further, there is no meandering portion in the semiconductor layer and therefore, deterioration by heat generation or the like can be restrained.
Further, the distance between the source and the drain is shortened compared with that in the conventional example and therefore, the ON resistance can be reduced and the thin film transistor having high mobility can be fabricated.